1. Field of the Invention
The present invention relates to color video systems.
2. State of the Art
The signals received by the three cameras of any color video system are determined by the reflectance properties of the objects in the scene and the spectral distribution function of the illuminant. Since the information of interest to the observer is determined by the scene and not by the illuminant i.e. what kind of objects, characterized by their reflectance properties, are present in the scene, not how they are illuminated, it is desirable to correct the signals from the scene for the illuminant before the information is displayed on the CRT, illuminant-correction enables the signals, to be displayed by the CRT, to be in standardized form, determined by the scene, so that the color rendering problem at the CRT can be solved independently of the accidental and unknown light source that illuminates the scene to be rendered on the CRT. The correction must take place at camera level because only there information about the illuminant is available. Such a correction has also additional advantages. The signals from the cameras are contaminated by the properties of the accidental illuminant. In particular, different signals are produced when tungsten lighting or an illuminant like D.sub.65 is used for illuminating the same scene. Illuminant-correction enables a better definition of the signals to be transmitted so that e.g. less bandwidth is needed for their transmission.
It is generally accepted that the human visual system is able to discount the illuminant i.e. to correct the information from the outside world for the presence of the illuminant. It is observed that nature looks similarly at dawn and at sunset, though the spectral distribution function of the incident light is very different. For more than a century the contention is whether the effect, called color constancy, is to be explained on the basis of signal processing or that memory is the agent. In the latter case, implementation in an artificial system would require a large memory, probably not practical. The hypothesis in the former case is that, using information about the illuminant, it may be possible to obtain information about reflectance only i.e. to discount the illuminant. Illuminant-correction creates a more or less stable world with obvious advantages for detection purposes. Discounting the illuminant can be achieved if a transformation can be defined which transforms the illuminant-dependent signals from the cones to variables which would result if the illuminant were a given, fixed reference source. An the equal-energy spectral distribution E(.lambda.), see definition (1). The fact that E(.lambda.) will be taken as the reference source=1 below everywhere is not exploited so that any desirable reference source can be used instead, as long as it remains fixed. Any such transformation requires knowledge about the actual illuminant. Furthermore, since many different illuminants exist that are visually equivalent in the sense that their chromaticity coordinates are equal, the correction cannot be complete. At most, almost illuminant-independent variables can be produced. They correspond to certain, by definition illuminant-independent properties of reflectance, explaining color constancy. Information about these properties cannot be obtained exactly but can only be estimated in view of the fact that three signals do not contain sufficient information to recover the complete behavior of the reflectance as a function of wavelength, even if the illuminant were completely known. Since estimates are subject to uncertainty, we may associate an error bar with the estimate of the illuminant-independent property of reflectance, hence to the variable in question, due to the lack of knowledge of the visual system about both the illuminant and the reflectance as functions of wavelength. Suppose, in addition, that the illuminant varies over a set of well-behaved illuminants and that illuminant-correction is applied. Since the correction is necessarily incomplete, the error bar associated with the illuminant-independent property of reflectance is somewhat enlarged but that does not affect the face that such an error bar exists. If the variables correspond closely to perceptually meaningful variables, then it may be expected that the resulting problem is accounted for by the color metrics, employed by the human visual system. If the error bar is small, i.e. if all uncertainties the visual system has to cope with are small, then the just-noticeable differences of the signal employed by the human visual system can likewise be chosen to be small. On the other hand, if the error bar is not particularly small, then these just-noticeable differences must be chosen correspondingly larger. Thus, preferably, the almost illuminant-independent variables are to be chosen such they they correspond as much as possible with perceptually meaningful variables so that `errors` in illuminant-correction are ignored, due to the color metrics, leading to improved image quality. If the error bars of the variables could reasonably accurately be estimated, then the result could be exploited in the data transmission because it does not make sense to transmit variables with a greater precision than allowed by their definition, leading to a saving of bandwith. In present color video systems the normalization of the white point of the actual illuminant to that of a fixed illuminant e.g. D.sub.85 provides some illuminant-correction.